Selectively tuned ultraviolet optical filters and methods of use thereof

ABSTRACT

A novel, cost effective optical filter is employed for photosensors used in ultraviolet-based water purification systems. Optically tuned to specifically eliminate the non-germicidal wavelength polychromatic emissions from mercury lamps, this unique optical filter approach significantly reduces the cost in manufacturing reliable water purification systems employing ultraviolet light.

FIELD OF THE INVENTION

[0001] The present invention relates to improved cost-effectiveultraviolet optical filtering devices. More particularly, the presentinvention relates to selectively tuned ultraviolet optical filtersuseful in mercury vapor lamp based UV water purification systems.

BACKGROUND OF THE INVENTION

[0002] Purified water is essential not only for drinking purposes, butalso for numerous other applications such as, for example, drug and foodmanufacturing, semiconductor processing, critical cleaning applications,heat exchanger coolant use, purification of swimming pool water, etc.

[0003] Of particular concern in the water purification industry is thatof providing purified drinking water to third world countries.Significant challenges continue to exist in this area due to the needfor a high degree of purification and utmost reliability required toprevent water-borne diseases (cholera, typhoid, hepatitis, etc.) Inaddition, such water purification treatment processes must have thecapacity to produce high volumes of purified water at the lowestpossible cost.

[0004] Chemical, biological and physical treatment processes arewell-known and are capable of providing water of varying degrees ofpurity. One popular process though is the exposure of germ-laden waterto the germicidal wavelength of an ultraviolet source. Exposing flowingwater to the ultraviolet germicidal wavelengths of 200-300 nm alters anddamages a bacteria's DNA, thereby preventing its reproduction. DNAabsorbs ultraviolet light strongly in the ultraviolet spectrum centeredat 260 nm. Thus, the typical dominant 254 nm emission of a mercury vaporlamp has been employed for this purpose.

[0005] The U.S. Public Health Service requires that ultraviolet waterpurification equipment have a minimum 254 nm ultraviolet dosage of16,000 micro-watt-seconds per square centimeter. In order to insure thatthis minimum criteria is satisfied, an ultraviolet mercury lamp is oftenmonitored with an optically filtered silicon photosensor (which directlymeasures the 254 nm emission). If the silicon photosensor measures a low254 nm emission, a warning is activated to replace the substandardultraviolet lamp. Determining when an ultraviolet mercury lamp has agedto the point where its germicidal effectiveness is diminished iscritical.

[0006] Some existing expensive UV water purification systems are in usewhich employ ultraviolet enhanced photodiodes fitted with standardoptical bandpass filters to monitor the life of the mercury lamp. Theseoptical bandpass filters define the performance of the optical system byselecting the critical 254 nm emission, while optically blocking theremaining full UV/VIS/IR spectral region (200 nm to 1200 nm). Althoughsuccessful in their application, such standard optical filters are veryexpensive, are limited in their 254 nm performance, and have substandarddurability which limits their longevity, field lifetime and versatility.Further, these optical bandpass filters have poor resistance toenvironmental exposure (e.g. moisture and temperature) and, thus, needto be very carefully hermetically sealed within the housing of thephotosensor. Thus, such optical filters are not suitable inapplications, such as water purification in third world countries, whichrequire utmost reliability at the lowest possible cost.

[0007] Presently available optical filters used in water purificationsystems are expensive standard narrow bandpass filters centered at 254nm. The two general types are: MDM (Metal-Dielectric-Metal) filters andSolar Blind Filters.

[0008] MDM filters consist of transparent quartz (or similar) substratesoptically coated with alternating thin films of a soft dielectric (e.g.cryolite) and metallic aluminum. Disadvantages of MDM filters include:poor resistance to elevated temperature, extreme fragility (soft, easilyscratched optical coatings limits their use) which requires the coatingsto be protected with additional quartz substrates, thickness and sizeconstraints and extreme cost (approximately $88 per filter). Shown inFIG. 4 is the spectral behavior of a typical 254 nm MDM bandpass filter.Solar Blind Filters are multi-element devices manufactured withabsorptive glasses and optical crystals (e.g. nickel sulfate). Suchfilters are very sensitive to moisture and heat, are very thick (5-6 mm)and cost prohibitive (approximately $250 per filter). Shown in FIG. 5 isthe spectral behavior of a standard Solar Blind Filter. Both MDM andSolar Blind Filters are limited in their application because they mustbe mounted and sealed within a photodiode housing. Because ofmanufacturability, the filter sizes must be large and cover the fullclear aperture of the photodiode housing (see FIG. 3).

[0009] It would, thus, be desirable to provide improved optical filtersfor ultraviolet water purification systems that are capable of producinglarge volumes of highly purified water with utmost reliability and atthe lowest possible cost.

SUMMARY OF THE INVENTION

[0010] The present invention provides an improved method and apparatusfor the ultraviolet purification of liquids, particularly water, at asubstantially lower cost and greater reliability than currentlyavailable systems. More particularly, the present invention providesunique ultraviolet optical filters selectively tuned to eliminate thediscrete non-germicidal wavelength polychromatic background emissionsfrom mercury lamps.

[0011] In addition to the typical 254 nm emission of a mercury lamp,mercury lamps also have polychromatic background emissions at otherdiscrete wavelengths, e.g., 313 nm, 365 nm, 405 nm, 436 nm, 546 nm, 579nm, 1015 nm and 1140 nm (see FIG. 2). This is a unique characteristic ofthese types of lamps. These wavelengths do not contribute to waterpurification, but do interfere with the accurate optical monitoring ofthe mercury lamp life using a silicon photodiode. To monitor thecritical 254 nm mercury lamp emission, these wavelengths must beblocked. Whereas standard expensive MDM or Solar Blind 254 nm bandpassfilters fully block the entire UV/VIS/IR spectral regions, it is theunique and novel feature of this invention to block only these discretebackground wavelengths. In this way, the optical filter cost isdramatically reduced and the reliability improved.

[0012] The optical filters of the present invention provide a number ofadvantages over prior optical filters including, for example, very lowcost (less than $2 as opposed to $88 for MDM filters and $250 for SolarBlind Filters), extreme durability to high temperatures and moisture,superior scratch resistance, small sizes, improved optical performance,extended physical longevity, high imaging quality of transmittedradiation, and improved throughput of the transmitted critical 254 nmwavelength. Further, preferred filters of the invention do not requireany sealing from the ambient atmosphere and do not degrade over timewith exposure to ultraviolet irradiation.

[0013] The optical filters of the present invention may be fabricated byconventional optical coating technologies including, for example,physical vapor deposition (thermal evaporation employing electron-beamtechnology), ion assisted deposition, ion beam or magnetron sputtering,chemical vapor deposition or reactive ion plating.

[0014] The design and dimensions of the optical filters in accordancewith the present invention makes them particularly suitable for use inwater purification systems that employ ultraviolet enhanced photodiodesfitted with optical filters. In one preferred embodiment, the opticalfilters of the present invention comprise a substrate having opticalcoatings thereon. Such optical filters may suitably form the externalwindow of the photodiode. In a particularly preferred embodiment,optical coatings are directly deposited upon the photodiode surfaceitself, which provides particularly substantial cost savings (FIG. 1).

[0015] In accordance with one embodiment of the present invention, theoptical filter comprises a substrate with optical coatings deposited onone or both surfaces of the substrate. The substrate may be selectedfrom a wide variety of conventional optical substrates including, forexample, glass, plastic, fused silica, metal or the like. In preferredembodiments, the substrate is either a single thin fused silicasubstrate or an ultraviolet transparent glass substrate.

[0016] The coating material of the present invention may be a variety ofmaterials recognized by those skilled in the art including low and highrefractive index materials. Low refractive index (n_(L)) materialsinclude, for example, SiO₂, Al₂O₃, SiO, fluorides such as bariumfluoride and lanthanum fluoride, MgO, etc. Collectively, low-indexmaterials are sometimes referred to herein as “L”. Low-index materials(L) are defined to mean herein materials having a refractive index(20/D) of less than 2.0, more typically 1.8 or less such as 1.8 to 1.3.Common high refractive index (n_(H)) materials include, for example,TiO₂, ZrO₂, Ta₂O₅, and HfO₂. Collectively, these high-index materialsare sometimes referred to herein as “H”. High-index materials (H) aredefined to mean herein materials having a refractive index (20/D) of 2.0or greater. As known to those skilled in the art, the designation “20/D”indicates the refractive index values are as measured at 20° C. using alight source of the D line of sodium. In particularly preferredembodiments, the coating materials are thin films of ultraviolettransparent refractory metal oxide (e.g. hafnium oxide, zirconium oxide,silicon dioxide, etc.).

[0017] In some embodiments, rather than form the optical coatings on theabove-described substrate materials, the optical coatings may, ifdesired, be coated directly upon the photodiode active area.

[0018] The spectral design of the optical filter is tuned specificallyin accordance with its ultimate use. For ultraviolet water purification,the optical filter transmits effectively within the wavelengths thatcontribute to ultraviolet sterilization (centered at 254 nm) andselectively rejects those background discrete wavelengths in theUV/VIS/IR emission spectra of typical mercury lamps and which fallwithin the sensitivity region of photodiodes.

[0019] The specially tuned optical filters of the present invention,when used in connection with ultraviolet enhanced photodiodes, provideaccurate low-cost monitoring of the critical 254 nm emission required inwater purification procedures.

[0020] Other aspects of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a simplified cross sectional view of one embodiment ofthe photodiode assembly in accordance with the present invention.

[0022]FIG. 2 depicts graphically the typical spectral emission from amercury lamp.

[0023]FIG. 3 is a simplified cross sectional view of a typicalphotodiode assembly employing current technology.

[0024]FIG. 4 depicts graphically the spectral behavior of a typical 254nm MDM bandpass filter.

[0025]FIG. 5 depicts graphically the spectral behavior of a typicalsolar blind filter.

[0026]FIG. 6 depicts graphically the transmittance of the optical filterin accordance with the present invention.

[0027]FIG. 7 depicts graphically the tuned ultraviolet and visiblerejection of the optical filter in accordance with the presentinvention.

[0028]FIG. 8 depicts graphically the IR rejection of the optical filterin accordance with the present invention.

[0029]FIG. 9 shows a simplified cross sectional view of the opticalfilter coating design in accordance with one embodiment of the presentinvention.

[0030]FIG. 10 shows a simplified cross sectional view of the opticalfilter coating design in accordance with a second embodiment of thepresent invention.

[0031]FIG. 11 depicts graphically the optical performance of the opticalfilter in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Referring now to the various figures of the drawing, wherein likereference characters refer to like parts, there is shown various viewsof the photosensor/optical filter assemblies used in ultraviolet-basedwater purification systems, optical filters and graphical performancedata in accordance with the present invention.

[0033]FIG. 1 shows a simplified view of one embodiment of the spectrallytuned optical filter in accordance with the present invention. Thefiltered photosensor comprises a housing 1 (the photodiode “can”).Within the housing 1 is a standard ultraviolet enhanced photodiode 2,and one optical filter (3, 4 or 5). In one preferred embodiment, theoptical filter 3 (consisting of optical coatings on a discrete UVtransparent substrate) is mounted within the housing 1 directly onto theactive area of the photodiode 2. In other embodiments, the opticalfilter 3 may be located in alternate mounting position 4. As a costsavings, the filter may uniquely also be used as the photodiode housingwindow 5 as shown in FIG. 1. Greater cost savings is obtained when thefilter is of the smallest size (position 3). The greatest cost savingsis obtained when the optical coatings are deposited directly upon thephotodiode surface without the use of the discrete UV transparentsubstrate.

[0034] As shown in greater detail in FIG. 9, in one embodiment, theoptical filter 3 comprises a substrate 6 with one or more opticalcoating layers deposited thereon. The optical coating layers comprise aUV/VIS rejection coating 9. Optionally, an IR rejection coating 10 mayalso be necessary, based upon the responsivity of the photodiode 2 thatis utilized. For example, IR rejection coatings 10 would generally notbe required when IR wavelengths do not fall within the sensitivityregion of the photodiode that is used. On the other hand, IR rejectioncoatings 10 should be used when IR wavelengths do fall within thesensitivity region of the photodiode that is used.

[0035] The substrate 6 may be selected from a wide variety ofconventional optical substrates including glass, plastic, fused silica,metal or the like. The substrate 6 is preferably substantiallytransparent and thin. In preferred embodiments, the substrate 6 iseither a single thin fused silica substrate or an ultraviolettransparent glass substrate.

[0036] In a preferred embodiment, as shown in FIG. 10, the coatinglayers 7 may be directly deposited on the active surface of theultraviolet enhanced photodiode 2 with the photodiode 2 acting as thesubstrate. Again, these optical coating layers 7 comprise a UV/VISrejection coating 9 and, optionally, an IR rejection coating 10.

[0037] The optical coatings 9 and 10 may be either deposited on twosides of the substrate 6 or, alternatively, all on one side of thissubstrate 6. Likewise, the optical coating layers 7 may be may be eitherdeposited on both sides of the ultraviolet enhanced photodiode 2 or,most preferably all on one side of this ultraviolet enhanced photodiode2.

[0038] The optical coatings 7 may be fabricated of coating materialsrecognized by those skilled in the art including low and high refractiveindex materials. Low refractive index materials include, for example,SiO₂, Al₂O₃, SiO, fluorides such as barium fluoride and lanthanumfluoride, MgO, etc. Common high refractive index (n_(H)) materialsinclude, for example, TiO₂, ZrO₂, Ta₂O₅, and HfO₂. In particularlypreferred embodiments, the coating materials are thin films ofultraviolet transparent refractory metal oxide (e.g. hafnium oxide,zirconium oxide, silicon dioxide, etc.). In a particularly preferredembodiment, the optical filter comprises a multiplayer coating ofalternating layers of hafnium oxide and silicon dioxide.

[0039] Various manufacturing processes may be employed to deposit theoptical coating layers 7 including, for example: physical vapordeposition (thermal evaporation employing electron-beam technology), ionassisted deposition, sputtering, chemical vapor deposition or reactiveion plating.

[0040] Thicknesses of coating layers applied in accordance with thepresent invention will typically vary from tens of nms per layer tohundreds of nms per layer, depending on applications, as will beappreciated by those skilled in the art. The overall size of the opticalfilters of the present invention is preferably no greater than 2 mm² toenable direct mounting of the filters onto the photodiode activesurface. In the event that the optical filters are used as thephotodiode housing window, the optical filters are sized to correspondto the size of the photodiode housing window, which typically isapproximately 6 mm in diameter. As the size of the photodiode housingwindow varies, the size of the optical filter varies accordingly.Preferred thicknesses of the optical filters range from about 0.5 mm toabout 1 mm nominally.

[0041] The optical filters 3 of the present invention exhibitexceptional properties, particularly for use in ultraviolet waterpurification systems utilizing a mercury lamp. As discussed above,preferred optical filters 3 of the invention do not require any sealingfrom the ambient atmosphere (e.g. no epoxy or other encapsulation), donot degrade over time and exposure to UV irradiation, and offer superiortransmittance at the critical 254 nm wavelength. Such filters are usefulfor a wide variety of applications, particularly where currenttechnology bandpass filters are simply too expensive.

[0042] More specifically, preferred optical filters 3 of the inventioninclude filters that have a 254 nm optical transmittance of at leastabout 40%, more preferably at least about 70%, still more preferably atleast about 75% or 79%. Such transmittance is maintained over extendedperiods of exposure to ultraviolet radiation, e.g. exposure to radiationfrom a typical germicidal mercury vapor lamp at energy doses typicallyprovided by the following: positioning a filter of the invention fromabout 0.25 inches to 0.5, 1, 2, 3, 4, 5, 6, 7, or 8 inches away from agermicidal mercury vapor lamp that may be 10, 20, 50, 100, 200, or 500watts, for at least 100 minutes of exposure to such a lamp, moretypically at least about 1000, 2000, 5000, 9000 or more hours ofexposure to such a lamp. That is, preferred filters of the invention donot degrade under such extended exposure to ultraviolet light.

[0043] More specifically, preferred optical filters 3 of the presentinvention are tuned specifically for ultraviolet water purification,and, as such, the optical filters 3 transmit effectively within thewavelengths that contribute to ultraviolet sterilization (centered at254 nm) and selectively reject those background discrete wavelengths inthe UV/VIS/IR emission spectra of typical mercury lamps and which fallwithin the sensitivity region of photodiodes. More specifically,preferred optical filters 3 have an optical transmittance of at leastabout 40%, more preferably at least about 70%, still more preferably atleast about 75% or 79% at a wavelength of about 254 nm. Further,preferred optical filters 3 of the present invention have an opticaltransmittance of no greater than 5% at wavelengths of 313 nm to 580 nmand 1000 nm to 1140 nm. Still further, preferred optical filters 3 ofthe present invention have an optical transmittance of no greater than1% average within these wavelength regions. Still further, preferredoptical filters 3 of the present invention have an optical transmittanceof no greater than 2% absolute at wavelengths of 313 nm, 365 nm, 405 nm,436 nm, 546 nm, 579 nm and 1015 nm.

[0044] The optical coating layers of the present invention exhibitexcellent durability to harsh environmental conditions and excellentsubstrate adherence. More specifically, coatings applied by methods ofthe invention have withstood 100 or more cycles of high temperature andaggravated humidity per MIL-STD-810E (standard tests; militaryspecification). Following such environmental exposure, the same coatingsmaintain excellent substrate adherence and pass the snap-cellophane testper MIL-C48497 (standard test; military specification).

[0045] Ultraviolet-based water sterilizing systems utilizing opticalfilters of the present invention may be used to produce large volumes ofhighly purified water with utmost reliability and at such a low costthat its widespread use in third world nations may now be possible.

[0046] All documents mentioned herein are incorporated by referenceherein in their entirety. The following non-limiting examples areillustrative of the invention (designs will vary depending upon theparticular refractive indices of the coating materials and coatingtechnologies employed). In the Examples, the optical coatings weredeposited using ion plating deposition, by procedures disclosed in U.S.Pat. Nos. 6,139,968 and 5,753,319.

EXAMPLE 1

[0047] A multilayer optical filter having layers of hafnium oxide andsilicon dioxide deposited on a synthetic fused silica substrate wasfabricated as follows:

[0048] A. A UV/VIS rejection coating was deposited on one surface of thesynthetic fused silica substrate as follows, (see also FIG. 9, referencenumber 9):

[0049] SUBSTRATE/0.30076H/0.12755L/(0.125L 0.25H0.125L)10/0.1216L/0.2857H/0.0904L/(0.163L 0.325H0.163L)12/0.118L/0.2954H/0.12159L/(0.2065L0.413H0.2065L)13/0.1542L/0.2648H/0.52797L/AIR

[0050] wherein:

[0051] Substrate: Synthetic Fused Silica (approximately 0.7 mm thick)

[0052] High Index Material (H): Hafnium Oxide

[0053] Low Index Material (L): Silicon Dioxide

[0054] 0.25H=1 Quarter Wave Optical Thickness of Hafnium Oxide

[0055] 0.25L=1 Quarter Wave Optical Thickness of Silicon Dioxide

[0056] Design Wavelength=333 nm

[0057] B. An IR rejection coating was then deposited on the oppositesurface of the synthetic fused silica substrate as follows, (see alsoFIG. 9, reference number 10):

[0058]SUBSTRATE/0.749H/0.405L/(0.39156L0.8275H0.39156L)9/0.3858L/0.80726H/0.2052L/AIR

[0059] wherein:

[0060] Substrate: Synthetic Fused Silica (approximately 0.7 mm thick)

[0061] High Index Material (H): Hafnium Oxide

[0062] Low Index Material (L): Silicon Dioxide

[0063] 0.25H=1 Quarter Wave Optical Thickness of Hafnium Oxide

[0064] 0.25L=1 Quarter Wave Optical Thickness of Silicon Dioxide

[0065] Design Wavelength=343 nm

[0066] The filter was produced by the ion plating deposition of hafniumoxide and silicon dioxide onto 2.0″ square synthetic fused silicasubstrates. The thickness of the synthetic fused silica substrate wasapproximately 0.7 mm.

[0067] Thicknesses of the halfnium oxide layers and of the silicondioxide layers were as shown above. After dicing, the overall size ofthe optical filter was approximately 2 mm×2 mm SQ. Other sizes producedincluded 0.240″ diameters.

[0068] The measured performance of the thus formed multilayer opticalfilter was tested with the following results, as graphically shown inFIG. 11. The measured %T at 254 nm is 79.4%. Further, the %T at thefollowing wavelengths are as follows:

[0069] %T=0.123% AVG (310 nm-580 nm); %T=0.23% AVG (1000 nm-1140 nm);%T=0.28% at 313 nm; %T=0.18% at 365 nm; %T=0.02% at 405 nm; %T=0.003% at436 nm; %T=0.035% at 546 nm; %T=0.56% at 579 nm; %T=0.175 at 1015 nm.

[0070] The foregoing description of the invention is merely illustrativethereof, and it should be understood that variations and modificationscan be affected without departing from the scope or spirit of theinvention as set forth in the following claims.

What is claimed is
 1. An ultraviolet optical filter comprising: at leastone substrate having a first surface and a second surface; and at leastone coating deposited on at least one surface of the substrate, wherebythe ultraviolet optical filter has an optical transmittance of at leastabout 40% at a wavelength of 254 nm and optical transmittance of nogreater than 5% for wavelengths of 313 nm, 365 nm, 405 nm, 436 nm, 546nm, 579 nm, 1015 nm and 1140 nm.
 2. The optical filter of claim 1,wherein the overall size of the optical filter is no greater than about2 mm².
 3. The optical filter of claim 1, wherein the substrate isselected from glass, plastic, fused silica or metal.
 4. The opticalfilter of claim 3, wherein the substrate is fused silica.
 5. The opticalfilter of claim 3, wherein the substrate is an ultraviolet transparentglass.
 6. The optical filter of claim 1, wherein the substrate is aphotodiode.
 7. The optical filter of claim 1, wherein the opticalcoating layer comprises one or more layers selected from SiO₂, Al₂O₃,SiO, fluorides such as barium fluoride and lanthanum fluoride, MgO,TiO₂, ZrO₂, Ta₂O₅, and HfO₂.
 8. The optical filter of claim 1, whereinthe optical coating layer comprises one or more metal oxide layers. 9.The optical filter of claim 1, wherein the ultraviolet optical filterhas an optical transmittance of no greater than 5% for discretebackground UV/VIS/IR wavelengths typical of the emission spectra of agermicidal mercury vapor lamp.
 10. An optical filter comprising: atleast one substrate having a first surface and a second surface; and atleast one coating deposited on at least one surface of the substrate,whereby the ultraviolet optical filter transmits effectively at 254 nmand rejects wavelengths of 313 nm, 365 nm, 405 nm, 436 nm, 546 nm, 579nm, 1015 nm and 1140 nm.
 11. An optical filter comprising: a multilayerstack of refractory metal oxides, deposited directly upon a photodiodeactive surface, whereby the optical filter transmits effectively at 254nm and rejects wavelengths of 313 nm, 365 nm, 405 nm, 436 nm, 546 nm,579 nm, 1015 nm and 1140 nm.
 12. The optical filter of claim 10 or 11,wherein the optical filter transmits at least 40% at a wavelength of 254nm.
 13. A method for forming an optical filter, the method comprisingthe steps of: depositing an optical coating layer on at least onesurface of a substrate, wherein the optical filter has an opticaltransmittance of at least about 40% at a wavelength of 254 nm andoptical transmittance of no greater than 5% for wavelengths of 313 nm,365 nm, 405 nm, 436 nm, 546 nm, 579 nm, 1015 nm and 1140 nm.
 14. Themethod of claim 13, wherein the substrate is selected from glass,plastic, fused silica or metal.
 15. The method of claim 13, wherein thesubstrate is fused silica.
 16. The method of claim 13, wherein thesubstrate is an ultraviolet transparent glass.
 17. The method of claim13, wherein the substrate is a photodiode.
 18. The method of claim 13,wherein the optical coating layer comprises one or more layers selectedfrom SiO₂, Al₂O₃, SiO, fluorides such as barium fluoride and lanthanumfluoride, MgO, TiO₂, ZrO₂, Ta₂O₅, and HfO₂.
 19. The method of claim 18,wherein the optical coating layer comprises one or more metal oxidelayers.
 20. The method of claim 18, wherein the optical filter has anoptical transmittance of no greater than 5% for discrete backgroundUV/VIS/IR wavelengths typical of the emission spectra of a germicidalmercury vapor lamp.